[llvm] ARM: Remove override of shouldRewriteCopySrc (PR #125219)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> As far as I understand this will start creating "illegal" copies that are not themselves supported (they go from high dregs+subreg to sregs, which isn't a valid copy). 

The code here is only trying to avoid potentially expensive cross register bank copies. The details of how a COPY is implemented later aren't so important, so long as the movs we end up with after expansion isn't more expensive 

https://github.com/llvm/llvm-project/pull/125219